Gas water heating problem

Gas house water heating. We use 9KG gas bottles to supply water heater. The gas water heater will not work when 9KG gas bottle gets to half full. Can anyone explain why this is happening? Thanks in advance

LPG is a liquid in the cylinder. It needs to boil to evolve off gas phase vapour to flow through to your appliance. The amount of gas that can be extracted is a function of the size of the cylinder and the surface area of the liquid.

Your water heater is asking for more gas than a 9 kg cylinder can supply. You can sometimes get away with a 9 kg cylinder on a W/H and sometimes (as you've found out) you can't.

Is it cold and frosty your way? This brings in another factor. NZ LPG is a mixture of propane and butane. Propane has a much lower boiling point than butane so it gets used up first. If you have mostly butane left in the cylinder and the outside temperature gets below freezing you won't get anything as butane has a boiling point of around -1°C

https://www.elgas.com.au/blog/2241-can-you-use-a-9kg-gas-bottle-instead-of-a-45kg

Happens to my 9kg BBQ bottle, but I just tip it upside down til it starts flowing.Started happening about 4yrs ago. Don't know whether they changed the mixture or not. But it is a pita.

We have 2 45kg bottles for our house hot water and cooktop. Does it automatically change bottles when one is empty ? It is showing "RED" on indicator but the lever/arrow is still facing the old bottle. Do I have to manually turn the lever over or will it do it automatically ? We have not run out of hot water yet.

If your running 2 x 45's always turn one off and run it dry, or it'll change over to the full one before it's completely empty. even with the lever pointing to the low one.

Maybe lift the bottle up so it's level with the regulator. we sometimes run a 4.5kg bottle when the 2 45's are empty, and it works just fine.

How's that going to make any difference? What's the physics in the height of the liquid changing the pressure of the vapour?

And in any case I suspect OP's installation will be one of those where the regulator screws into the cylinder like a BBQ.

I too leave one bottle turned off (2 x 45kg), the only thing worse than running out of water while soaping your hair in the shower, is running out and not having another bottle to turn on, if you you use the stupid auto changeover, and then have to perhaps wait days for next delivery.
Ps, i know to immediately turn water off, when it starts getting cold patches mid shower, as that clearly indicates burner is cutting off, likely due to lack of gas. So i finish soaping etc with water off for next 2 minutes, then turn on again for a fast rinse, which system always manages to supply, after that off cycle(bottle has regained a little pressure).
All very well for some folk to spout about the red flag indicator on system, but who the heck makes a habit of going and looking at it regularly, especially if like me you usually only use 2-3 bottles a year(but varies a fair bit some seasons). I have a notice/instruction on my cover, for delivery guy to never turn both on, as most seem to automatically do so.

In a pinch, if desperate to get gas out of a too cold bottle(which likely is due to being way too small for the duty you are asking of it), continuously running a cold hose over it, will warm it enough to go again.

you have to open the other bottle tap if it is not already open and flick the arrow and the pressure should turn the indicator green again.